Interpretive Summary: Azalea web blight is an annual problem on some evergreen azalea cultivars grown in containerized nursery production in the southern and eastern United States. The pathogen, binucleate Rhizoctonia species, is undetectable to the human eye and is spread on new stem growth collected for vegetative propagation. The most effective way to eliminate the fungus is to submerge azalea stem cuttings in 122°F water for 21 minutes. The objective of this research was to evaluate the risk of hot water treatment on tissue damage and root development of twelve azalea cultivars. A 20 minute hot water treatment was safe to all 12 cultivars. It was shown that stem cuttings would develop minor to severe damage if unintentionally submerged for 40 to 80 minutes. The information will be beneficial to research and extension scientists and commercial ornamental plant producers.

Technical Abstract:
Azalea web blight is an annual problem on some evergreen azalea cultivars grown in containerized nursery production in the southern and eastern United States. The binucleate Rhizoctonia species, which cause the disease, are spread on new shoot growth harvested for propagation. Rhizoctonia can be eliminated by submersing stem cuttings in 122°F (50°C) water for 21 minutes. The objective of this research was to evaluate the risk of hot water treatment on tissue damage and root development of twelve azalea cultivars ('Conleb' (Autumn Embers™), ‘Fashion’, ‘Formosa’, ‘Gumpo White’, ‘Hardy Gardenia’, ‘Hershey Red’, ‘Macrantha Pink’, ‘Midnight Flare’, ‘Red Ruffles’, ‘Renee Michelle’, 'Roblel' (Autumn Debutante™) and ‘Watchet’). When stem cuttings were submerged in 122°F (50°C) water for 20 minutes, 11 of the 12 cultivars rooted well. ‘Fashion’ had slightly less root growth, but root systems were healthy and likely would have achieved comparable growth within a few weeks. When stem cuttings were submerged in 122°F water for 40, 60, and 80 minute, which is longer than needed to kill the pathogen, cultivars varied in sensitivity to damage from no damage to death of the cutting.